1. The Field of the Invention
The present invention relates to sputtering methods in the manufacture of semiconductor devices. More particularly, the present invention is directed to novel processes for sputtering with multiple ion species for improved bottom coverage and improved sputter rate in the manufacture of semiconductor devices.
2. The Relevant Technology
Various types of sputtering processes, including RF and DC sputtering, magnetron assisted sputtering, triode sputtering, ion beam sputtering, and others, have found wide application in the manufacture of semiconductor devices for deposition and for other applications.
Sputter deposition is one of the most economical alternatives for depositing many types of films. With particles of sputtered material approaching a substrate at various angles of incidence, sputtering can provide films having excellent uniformity.
As integrated circuits have become increasingly dense, however, the multi-directional flux of deposition material typically produced by sputtering has become a disadvantage for certain processes.
Contact and via plugs and other structures of highly dense integrated circuits may have aspect ratios as high as 5:1 or more. Such structures are generally formed by filling a trench or hole previously defined in an underlying layer or layers with materials deposited by sputtering or CVD processes. The multi-directional flux of typical sputtering processes can cause the trench or hole to be closed off at the top thereof without adequate filling of the bottom thereof, resulting in a xe2x80x9ckeyhole.xe2x80x9d
This problem is illustrated in FIG. 1. FIG. 1 is a partial cross-section of a partially formed integrated circuit device. A hole 16 has been previously formed in an underlying layer 12. A layer 14 of a deposited material is being sputter deposited over layer 12. Sputtered atoms of the deposited material approach layer 12 at various angles of incidence, including for example along the directions indicated by arrows A, B, C. Sputtered atoms approaching layer 12 in the direction of arrow B result in a buildup 18 of layer 14 on the right side of hole 16. Sputtered atoms approaching layer 12 in the direction of arrow C result in a buildup 20 of layer 14 on the left side of hole 16. Buildup 18 and buildup 20 eventually approach one another, closing off hole 16 and leaving a keyhole-shaped portion of hole 16 unfilled.
FIG. 2 schematically represents the standard solution to the problem of insuring adequate bottom coverage of high-aspect ratio features such as hole 16 illustrated in FIG. 1. A target 22 of a material to be sputtered is placed some distance from a substrate 26. A plasma 24 is formed, and ions from plasma 24 are accelerated toward target 22, sputtering target 22, producing a multi-directional flux of sputtered atoms of the material of target 22. A collimator 28 is placed between target 22 and substrate 26. Collimator 28 functions as a screen or filter, preventing sputtered atoms of target material approaching substrate 26 at large angles of incidence from reaching substrate 26. Such sputtered atoms are deposited on collimator 28 instead.
Sputtering with a collimator as illustrated in FIG. 2 has certain drawbacks. Deposits of the target material build up on the collimator, requiring frequent cleaning with associated downtime. The collimator reduces the deposition rate at substrate 26, requiring longer processing time for a given deposition thickness. The collimator also can exacerbate non-uniformities in the sputtering process, resulting in wider thickness variations within the deposited film. Decreasing the aspect ratio of the collimator reduces these problems, but reduces the collimator""s effectiveness. Hence an improved method of sputter deposition for high-aspect ratio layers is needed.
An object of the present invention is to provide a method of sputter deposition for high-aspect ratio layers providing improved bottom coverage.
A further object of the present invention is to provide a method of sputter deposition for high-aspect ratio layers providing increased deposition rates.
A still further object of the present invention is to provide a method of sputter deposition for high-aspect ratio layers allowing decreased collimator aspect ratio.
Another object of the present invention is to provide a method of sputter deposition for high-aspect ratio layers allowing deposition without a collimator.
A further object of the present invention is to provide a method of sputter deposition having increased time between required collimator cleanings.
A still further object of the present invention is to provide a method of sputter deposition employing two or more ionized species to achieve any of the forgoing objectives.
In accordance with the present invention, the perpendicularity of a flux of sputtered atoms to a substrate surface is increased by bombarding a target with both a low and a high mass ion species.
Where the low mass ion species is the typical sputtering species for a given target, the low mass species preferably predominates. The high mass ions pack the target nearer to the target surface than the typical packing by the low mass ions, causing target atoms ejected by the impact of low mass incident ions to have a higher probability of ejection in a direction perpendicular or nearly perpendicular to the target surface.
Where the high mass ion species is the typical sputtering species for a given target, the high mass species preferably predominates. The low mass ions pack the target at a deeper depth than the typical packing by the high mass species, resulting in a higher sputter rate than achievable with either the high or low mass species alone.
As an alternative, three or more ion species may be employed, with high, medium and low mass species included. The medium mass species packs the target at a certain depth, while the low mass species packs the target at a deeper depth and the high mass species packs the target at a shallower depth, resulting in increased target packing with increased sputter rate and increased perpendicularity of the sputtered flux.
In the above processes, a sputtering species having a lower ionization energy than a sputtering species with which it is employed allows a reduced pressure plasma as compared to a plasma composed solely of the sputtering species with which it is employed, resulting in less scattering of the sputtered flux, thereby improving the directionality of sputtered target atoms reaching the substrate. A low ionization energy species may also be employed to assist in striking a plasma, but then may be removed from the plasma before deposition.
These and other objects and features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.